1. Field of the Invention
The present invention relates to a treatment device of water to be treated (hereinafter referred to as “for-treatment water”) containing a fluorine content or for-treatment water containing nitrogen compounds and the like discharged from, for example, a semiconductor plant or the like.
2. Description of the Related Art
Heretofore, it is well known that one of causes for eutrophication of rivers and lake is the presence of nitrogen compounds. A large amount of the nitrogen compounds exist in waste water such as general household domestic waste water or plant waste water, but it is difficult to purify the water, and an effective measure cannot be taken in the present situation.
In general, as a treatment of the nitrogen compounds, an aerobic, anaerobic and biological treatment is performed, and this treatment is performed by two steps of a nitrification step of converting ammonia nitrogen into nitrite nitrogen, and further converting nitrite nitrogen into nitrate nitrogen, and a denitrification step of converting nitrate nitrogen into a nitrogen gas. Therefore, two reaction tanks are required and a treatment time is long, and hence, there is a problem that a treatment efficiency drops. Especially, since a reaction efficiency of the nitrification step of oxidizing ammonia to form nitrate nitrogen is low, a large reaction tank is required.
To solve the problem, during the treatment of the nitrogen compounds, an electrolysis treatment is performed in which a noble metal material such as platinum, iridium or palladium is used for an anode, and a current is applied to waste water as a treatment object to treat ammonia nitrogen, nitrite nitrogen and nitrate nitrogen in the waste water into the nitrogen gas (e.g., see Japanese Patent Application Laid-Open No. 54-16844).
However, in a case where for-treatment water containing highly concentrated nitrogen compounds is treated, there is a problem that enormous power cost is required. Therefore, when the for-treatment water containing the highly concentrated nitrogen compounds is reduced down to the nitrogen gas by the electrolysis treatment only, there is a problem that a remarkable cost burden is imposed.
To solve the problem, examples of a conventional device to treat the for-treatment water containing the nitrogen compounds include water treatment devices described in Japanese Patent Application Laid-Open Nos. 2004-330182 and 2004-122032. Each of these water treatment devices includes electrolysis treatment means and biological treatment means, and the water treated by the electrolysis treatment means is further treated by the biological treatment means to thereby effectively treat the nitrogen compounds remaining in the treated water. In consequence, it is possible to suppress a cost rise caused in the case of treating the water by the electrolysis treatment only. Moreover, it is possible to prevent a treatment efficiency drop from being caused in the case of treating the water by the biological treatment means only.
On the other hand, examples of the for-treatment water containing the nitrogen compounds as described above include hydrofluoric acid waste water discharged from a semiconductor manufacturing plant and the like. The hydrofluoric acid waste water is waste water containing hydrofluoric acid used in an etching step of a semiconductor manufacturing process, hydrofluoric acid is referred to as buffered hydrofluoric acid, and besides hydrofluoric acid, a highly concentrated hydrofluoric acid solution is used which is prepared using ammonium fluoride, nitric acid, a hydrogen peroxide solution or the like with a predetermined concentration.
Therefore, such a hydrofluoric acid solution containing the highly concentrated nitrogen compounds has a problem that the waste water containing hydrofluoric acid cannot be treated by the water treatment device only in which the electrolysis treatment and the biological treatment are combined to treat the nitrogen compounds as described above. Since the waste water containing hydrofluoric acid cannot be released to the environment as it is, the waste water has to be treated as an industrial waste. This is regarded as an important problem in respect of not only environment adaptability but also treatment cost.
Moreover, in each of the water treatment devices described above, after the electrolysis treatment is performed, the biological treatment is performed. In the biological treatment, a large single tank contains therein heterotrophic bacteria to reduce nitrate nitrogen or nitrite nitrogen into the nitrogen gas. The electrolytically treated water is fed to the tank to thereby reduce nitrate nitrogen or nitrite nitrogen remaining in the treated water. In this case, even as to the electrolytically treated water, the concentration of the nitrogen compounds in the treated water largely fluctuates with the concentration of the nitrogen compounds in raw water, that is, waste water. Therefore, in accordance with the concentration of the for-treatment water, the fluctuation of a load enlarges with respect to the bacteria in the tank for use in the biological treatment. Especially, in a case where as compared with the concentration of the nitrogen compounds in the treated water at the end of the previous treatment, the nitrogen compound concentration of the next treatment is remarkably high, a state of the load on the bacteria largely fluctuates. When this results in an overload state with respect to the bacteria, the drop of the treatment efficiently is incurred.
Therefore, the present invention has been developed to solve the conventional technical problems, and a water treatment device is provided which can treat for-treatment water containing fluorine components to bring the water into a state adaptable to the environment. The device can also treat nitrogen compounds without being influenced by a concentration or an amount of for-treatment water containing the nitrogen compounds.